The present invention relates to multilayer coextruded light-reflecting films which have a narrow reflection band because of light interference and contain a layer of naphthalate-based polyester.
Iridescent multilayer films are composed of a plurality of generally parallel layers of transparent thermoplastic resinous material in which the contiguous adjacent layers are of diverse resinous material whose index of refraction differs by at least about 0.03. The film contains at least 10 layers and more usually at least 35 layers and, preferably, at least about 70 layers.
The individual layers of the film are very thin, usually in the range of about 30 to 500 nm, preferably about 50-400 nm, which causes constructive interference in light waves reflected from the many interfaces. Depending on the layer thickness and the refractive index of the polymers, one dominant wavelength band is reflected and the remaining light is transmitted through the film. The reflected wavelength is determined by the sum of the optical thickness of a pair of layers.
The quantity of the reflected light (reflectance) and the color intensity depend on the difference the two refractive indices, on the ratio of optical thicknesses of the layers, on the number of layers and on the uniformity of the thickness. If the refractive indices are the same, there is no reflection at all from the interfaces between the layers. In multilayer iridescent films, the refractive indices of contiguous adjacent layers differ by at least 0.03 and preferably by at least 0.06 or more. For first order reflections, reflectance is highest when the optical thicknesses of the layers are equal, although suitably high reflectances can be achieved when the ratio of the two optical thicknesses falls between 5:95 and 95:5. Distinct color reflections are obtained with as few as 10 layers. However, for maximum color intensity, it is desirable to have between 35 and 1000 or more layers. High color intensity is associated with a reflection band which is relatively narrow and which has high reflectance at its peak. It should be recognized that although the term xe2x80x9ccolor intensityxe2x80x9d has been used here for convenience, the same considerations apply for the invisible reflection in the ultraviolet and infrared ranges.
The multilayer films can be made by a chill-roll casting technique using a conventional single manifold flat film die in combination with a feedblock which collects the melts from each of two or more extruders and arranges then into the desired layer pattern. Feedblocks are described for instance in U.S. Pat. Nos. 3,565,985 and 3,773,882. The feedblocks can be used to form alternating layers of either two components or more (e.g. ABABAB . . . , ABCABC . . . or ACBCACBC . . . ). The very narrow multilayer stream flows through a single manifold flat film die where the layers are simultaneously spread to the width of the die and thinned to the final die exit thickness. The number of layers and their thickness distribution can be changed in inserting a different feedblock module. Usually, the outermost layer or layers on each side of the sheet are thicker than the other layers. This thicker skin may consist of one of the components which makes up the optical core, may be a different polymer which is utilized to impart desirable mechanical, heat sealing, or other properties, or may be a combination of these.
Examination of iridescent films of desirable optical properties revealed deficiencies in certain mechanical properties. For example, the adhesion between individual layers of the multilayer structure may be insufficient, and the film may suffer from internal delamination or separation of layers during use. The iridescent film is often adhered to paper or board for its decorative effect, and is then used for greeting cards, cartons, wrapping paper and the like. Delamination of the film is unsightly and may even lead to separation of the glued joints if carton. In addition, the solvent resistance and heat stability of such films are not as great as desired for widespread utilization.
In U.S. Pat. No. 4,310,584, these deficiencies are significantly overcome by using a thermoplastic terephthalate polyester or copolyester resin as the high refractive index component of the system in which two or more resinous material form a plurality of layers. While a substantial improvement was realized, it also required the use of two polymers from significantly different polymer families. That fact, in turn, means that there are inherent significant differences between the two polymers and their relative adhesion to each other, chemical resistance, toughness, etc. As a result, the film itself is generally no better with regard to a particular characteristic than the weaker or poorer of the polymers employed. If two polymers closely related were employed in order to maximize relative adhesion to one each other, or toughness, or chemical resistance, etc., the polymers involved did not have a sufficient difference in refractive index so as to create the desired iridescent color.
Schrenk and Wheatly (Co-extruded Elastomeric Optical Interference Film, Antec ""88, 1703-1707) have reported the preparation of a multilayer light reflecting film co-extruded from two thermoplastic elastomers. The film which had one thermoplastic elastomer based on nylon and the other based on urethane, exhibited reversible changes in reflection spectra when deformed and relaxed. That is, this very specific combination had the ability of stretching without losing appearance characteristics. This type of films has been described in more detail in U.S. Pat. No. 4,937,134.
U.S. Pat. No. 5,089,318 discloses that further improvements in adhesion, solvent resistance and the like can be obtained by employing a thermoplastic elastomer (TPE) as one of the resinous materials. Such materials are copolymers of a thermoplastic hard segment such as polybutyl terephthalate, polyethylene terephthalate, polycarbonate, etc., and a soft elastomeric segment such as polyether glycols, silicone rubbers, polyetherimide and the like.
While prior art structures represented significant improvement in the areas of delamination resistance and better solvent stability, there were still some limitations with regard to these properties. In addition, iridescent films of the prior art still had deficiencies relative to their temperature stability, tensile strength and UV stability. The present invention surprisingly provides significant improvements over current known structures with regard to these properties.
It is, therefore an object of the invention to provide a transparent thermoplastic resinous laminate having good heat and solvent stability, good tensile strength, good delamination resistance, and good UV stability.
In one embodiment, the present invention provides a transparent thermoplastic resinous laminate film of at least 10 very thin layers of substantially uniform thickness, said layers being generally parallel, the contiguous adjacent layers being of different transparent thermoplastic resinous materials of which one is a naphthalate-based polyester or copolyester resin, the contiguous adjacent layers differing in refractive index by at least about 0.03.
In another embodiment, the present invention provides a transparent thermoplastic resinous laminate film of at least about 70 very thin layers of substantially uniform thickness, said layers being generally parallel, the contiguous adjacent layers being of different transparent thermoplastic resinous materials of which one is a polyethylene naphthalate polyester or copolyester, and the other is a polybutylene terephthalate polyester or copolyester, wherein the outermost layers are polybutylene terephthalate polyester.
Other objects and advantages of the present invention will become apparent from the following description and appended claims.
It has now been found that the objectives of this invention are realized by employing a naphthalate-based polyester or copolyester resin as a component in the contiguous adjacent layers in the optical core of a transparent thermoplastic resinous laminate film. Preferably, the naphthalate-based polyester or copolyester is based on naphthalene dicarboxylate. Examples of usable polyester resin include polyethylene naphthalate and polybutylene naphthalate. Examples of usable copolyesters include copolyesters comprising ethylene naphthalate and/or butylene naphthalate. Preferably, the copolyester consists of ethylene naphthalate and butylene naphthalate.
The iridescent film of the present invention can be obtained by coextruding the naphthalate-based polyester or copolyester resin with a different transparent thermoplastic resin which is selected to differ in refractive index by at least 0.03 and preferably by at least 0.06. Among the other resinous materials which can be used are transparent thermoplastic polyester or copolyester resins characterized by a refractive index of about 1.55 to about 1.61. Examples of usable thermoplastic polyester resins include polyethylene terephthalate (PET) which is made by reacting either terephthalic acid or dimethyl terephthalate with ethylene glycol; polybutylene terephthalate (PBT) which is made by the catalyzed combination of 1,4-butanediol with either terephthalic acid or dimethyl terephthalate; and the various thermoplastic copolyesters which are synthesized using more than one glycol and/or more than one dibasic acid. PETG polyester, for example, is a glycol modified PET made from ethylene glycol and cyclohexanedimethanol (CHDM) and terephthalic acid; PCTA copolyester is an acid-modified copolyester of CHDM with terephthalic and isophthalic acids. Additional other resinous materials that can be coextruded with the naphthalate-based polyester or copolyester resin are listed in Table 1.
The iridescent film of the present invention can also be obtained by coextruding the naphthalate-based polyester or copolyester resin with a different transparent naphthalate-based polyester which is selected to differ in refractive index by at least about 0.03 and preferably at least 0.06.
The outermost layers of the iridescent film of the present invention can be the same or different from resins in the optical core. For example, the outermost layers can comprise a polyester or copolyester resin such as polybutylene terephthalate polyester or glycol modified polyethylene terephthalate like PETG polyester.
The number of layers in the iridescent film of the invention is at least 10 layers, preferably at least 35 layers and more preferably at least about 70 layers.
A preferred combination in accordance with this invention involves an iridescent film having the contiguous adjacent layers in the optical core being of different transparent thermoplastic resinous materials of which one is polyethylene naphthalate polyester or copolyester, and the other is polybutylene terephthalate polyester or copolyester, wherein the outermost layers are polybutylene terephthalate or PETG polyester.
The delamination resistance of a film is tested by restraining one surface of the film with adhesive tape. A second piece of adhesive tape is applied to the other surface of the film. This second piece of tape is then pulled away and any indications of delamination is noted. If no delamination is observed, the tape is reapplied and the test repeated until failure is noted. Different tapes with different tack levels can be used to more fully differentiate between various film structures. Additionally, the film sample being tested can be immersed in solvent prior to testing or may be scored to provide a more severe form of this test. The number of pulls to failure using a particular tape is typically recorded. A description of this test method can be found in U.S. Pat. No. 5,089,318.
To test the solvent resistance of the film, samples of the film are immersed in the challenge solvent. The sample is observed for any color change, for the time at which the solvent begins to affect the iridescent color of the film and the nature of the color change. The time to initial color change and the nature of the color change at set time intervals are recorded. This test is typically run for a period of seven days with observations taken throughout the seven-day period. At the end of the seven days, the film sample is removed from the challenge solvent and allowed to dry for twenty-four hours. After the twenty-four hour drying period, the tester notes how the iridescent film color has changed. This data is referred to as the xe2x80x9cColor Recoveryxe2x80x9d.
To test the heat shrinkage of the film, a 2xe2x80x3xc3x972xe2x80x3 piece of film is cut from the film in question. The color of this piece is measured using a spectrophotometer. Data measured include the dominant wavelength (DWL), peak wavelength (PWL), and % peak reflection of the sample. The test specimen is then placed in an oven at the test temperature for a period of fifteen minutes. The sample is then removed from the oven. The sample is measured using a ruler to determine the percentage of shrinkage experienced by the film. The color of the sample is re-measured and the changes in DWL, PWL and peak reflection are recorded. The color of the sample after heat exposure is also compared to the original color of the test material by the person performing the test. Using all of this data, the temperature at which the film color begins to change is determined. This value is referred to as the xe2x80x9cColor Shift Temperaturexe2x80x9d.